Audiometer



Sept. 30, 1941.

H. w. KOREN 2,257,262

AUDIOMETER Filed June 7, 1939 2 Sheets-Sheet 2 INVENTOR H. W. KOQEN s wkmymszu ATTORNEY Patented Sept. 30, i941 AUDIOMETEB Heiman W. Koren, Bronx, N. Y., assignor to Sonotone Corporation, New York, N. Y., a corporation of New York Application June 7, 1939, Serial No. 277,882

Claims- This invention relates to audiometers or instruments for measurement of the hearing sensitivity ofan individual. V

Among the objects of the invention is an audiometer in which an electric oscillation gen- K 'erator capable of generating pure tone frequencies'of the principal audible frequency range is combined with an attenuator which regulates the intensity of the oscillations impressed on the receiver so that the operation of the frequency selector automatically controls the output delivered by the attenuator to the receiver at each selected frequency to enable accurate regulation of the sound intensity impressed by the receiver of the hearing organ with a single attenuation scale; an audiometer in which the frequency selector arrangement is combined with the attenuation arrangement to assure that the sensation level of the sound impressed by the receiver of the hearing organ remains substantially constant while the frequency of the tone is varied over a selected part of the audible frequency range; an audiometer in which an attenuator is automatically controlled by the operation of the frequency selector so as to permit the required full exploration of the loss of useful hearing ofa deafened person in the midfrequency range of audible hearing in which the in the audiogram of Fig. 2 wherein the lower curye marked Threshold of audibility and the upper curve marked Threshold of feeling give th limits of the hearing ability of an average normal person. The lower curve shows for each frequency the intensity of the sound in decibels, above an arbitrarily chosen level of watts per square centimeter, at which the tone is barely audible. The upper curve gives for each frenormal range of hearing sensitivity is highest, as

well as in the lower and upper parts of the audible frequency range, in which the range of normal hearing sensitivity is much lower, without damage to the receiver and without requiring an oscillation generator of excessively large output capacity.

The foregoing and other objects of the inven' tion will be best unde'rstood'from the following description of an exempliflcation thereof, reference being had to the accompanying drawings, wherein- Fig. 1 is an elevationalview of the front panel of an audiometer of the invention, parts of the panel being broken away to show some of the operating parts;

Fig. 2 is a diagram showing the hearing characteristics of a normal person; and

Fig. 3 is a diagram showing the circuit connections and the cooperative relationship of the frequency selector and the attenuation regulator of the audiometer exemplifying the invention.

A satisfactory audiometer mustenable accurate measurement of the hearing sensitivity of an individual over the principal frequency range of audible sound. The design of a satisfactory audiometer must take into consideration the hearing characteristics of a person having normal hearing. as determined by accurate measurement of the hearing sensitivity of a group of normal individuals having good hearing.- Typical results of such measurements are shown quency the intensity of the sound at which it is so loud thatit produces a sensation of feeling 7 and becomes painful. The points at which these two curves intersect give the lower and upper limits of audibility. These curves are useful in making measurements of hearing impairments and in computing the percentage of hearing or the percentage of hearing loss of a person of impaired hearing. Thus, for instance, taking the toneof 102-1 cycles at which the total range of hearing sensitivity between the lower and upper thresholds of hearing for anormal person is 132 db., a person having 'a hearing loss of 66 db. at this tone is said to have a 50% hearing loss at the given tone.

As shown in Fig. 2, the useful range of hearing at each frequency that is the number of decibels between the threshold of hearing and the threshold of pain varies irregularly over the frequency range. The frequency range which is important for hearing, and in which the range of hearing sensitivity is greatest lies between about 100 and 8000 cycles. In the mid portion of this frequency range, between about 500 and 2000 cycles, the range of hearing sensitivity is a maximum, amounting to about 130 db. above the threshold of hearing, the range of hearing sensitivity going down for the lower and higher parts of the irequency range, being only about 115 db. at 256 and &096 cycles, and only about 90 db. at 128 and 8192 cycles. In other words, taking the hearing sensitivity of a person having normal hearing as I a standard, the maximum possible lossof hearing in themid-frequency range is about 128 db., and the maximum possible loss of hearing in the upper and lower parts of the frequency range is only about 90 db.

A satisfactory audiometer for determining the hearing impairments of deafened persons should be able to measure loss of hearing in the principal audible frequency range between about 128 and 8192 cycles. In addition, a good audiometer should be able to impress at each frequency within the testing range, that is important for the exploration of the degree of deaf ness of an individual, a pure tone, and the .intensity of the tone should be accurately measurable. A change in sound intehsity by one decibel is just about the minimum change detectible by the human ear. However, on account of the uncertainty of judgment of an average individual,

one decibel change in sound intensity is of no great significance. For most practical mufposes,

" capable of determining the hearing ability in steps of five decibels.

A simple and accurate and practical audiometer exemplifying the invention which meets these requirements is shown diagrammatically in Fig. 3, Fig. 1 illustrating the panel and some features of the mechanism of the audiometer.

As'shown in Fig. 3, the audiometer comprises an oscillation generator which is, controlled by a frequency selector dial Ill, and an attenuator which is regulated by dial ll. V

The oscillation generator comprises an oscillator tube l2 having a cathode IS, a plate l4 and a control grid l5; twopush-pull connected ainplifier' tubes J6, each having a cathode I 3, a plate l4, a control grid I5, and a screen grid [1; and

a network of inductances, condensers and associated elements for enabling selective generation of the required pure tone frequencies.

In-the practical embodiment of the invention, the oscillator tube I2 is a type 31 triode, and the amplifier .tubes l6 are type 43 pentodes. The tubes are energized from a conventional 110- volts electric supply line 20 through a power pack 2| which is connected to the supply line by a power switch 22. The power pack 2| comprises a rectifier 23, a filter choke 24, and two filter condensers 25 connected in shunt to the grounded lead of thefllter. A meter 26 and rheostat 21 enables accurate adjustment of the direct current voltage delivered at the positive filter terminal +B, to assure prcper'operation of the oscillation generator. 'I'hefour heaters of the cathodes H are shown energized through a parallel supply circuit including a rheostat 2|. a

signal light 2! and a pilot light Ill.

The plate l4 of the oscillator tube I2 is connected through a choke coil II of the B+ terconnected in series with a self-biasing resistor 32 to the ground, a by-pass condenser 33 being connected parallel to the resistor 32. The plates of the amplifier tubes I are connected in pushpull to the primarywinding of an output transformer 35, the mid-point of which is connected through a lead 36 to the plate supply terminal The other type of oscillation generator uses a resonant oscillating circuit which fixes the frequency at which the voltage feed back to the grid of the oscillator tube is sufiicient for generating sustained oscillations. Such fixed frequency oscillation generators are extremely stable and give a signal of a high degree of purity free from harmonics or overtones which may cause errors in the audiometric tests and are thus a serious objection in audiometry. Thus, for instance, if an audiometer that has a distorted tone of 1000 cycles, in which are present harmonics of 2000 and 3000 cycles, is used for testing the hearing of a person who is very deaf to a tone of 1000 cycles, but who has a substantially normal hear-' ing at 2000 cycles, the subject may indicate that v he heardthe 1000 cycle tone when in 'reality he is hearing the harmonic tone of 2000 cycles. Accordingly, special care must be taken that the tube oscillator used in an audiometer should generate pure sinusoidal oscillations; and that the amplifier should operate without distortion so that harmonics may not be introduced; and also that the receiver should be capable of transforming the electrical energy into sound energy without distortion, that is, without producing sounds which have frequencies other than those of the electrical oscillations impressed upon the receiver. It is only when these conditions are met that one can be sure that the hearing of a sub- I ject is tested actually'at the frequency indicated by the frequency selector.

To assure such pure tones, the oscillation generator of the audiometer described in connection with Fig. 3 is designed to operate with fixed tuned circuits for producing pure tone oscillations of the different frequencies important in making audiometric tests, namely, the frequencies separated by an octave in the rangev between 128 to 8192 cycles per second. To this end, the oscillator tube l0 isarranged to selectively cooperate with two tuning inductances Ll, L2, a bank of tuning condensers CI to Cl, and a bank of feed-back resistors Rl to R1, shown in Fig. -3, as vertically V aligned with the corresponding tuning condensminal of the power pack, and .its cathode I3 is +B,'and the cathodes of the amplifier tubes it are connected through a resistor 31 to ground.

Accurate audiometric measurements require oscillators which produce very pure tones. There are two types of vacuum tube oscillator circuits capable of generating the different audio-frequencies required for audiometric measurements. The beat frequency oscillation generators, in which the test tone is produced by the difference of two oscillations of different frequencies far above the audible frequency range, enable variation of the frequency over the audio-frequency range by changing the frequency of one of the super-audible frequencies. However, beat frequency oscillators are subjected to frequency drifts since a variation of one-tenth of one per cent in the frequency of one of the component oscillators may mean a change of 100 cycles in the audible tone. As a result, precautions must be taken in the design of such beat frequency oscillators, and they require special attention. and

erswith which they are selectively interconnected ferent frequencies. Tocomplete an oscillating circuit for a given frequency, for instance, for 1024 cycles, the'end blade E of the switch I024 is flexed to actuate its other aligned contact blades to the closed position, as indicated in Fig. 8. tlllfil'tby establishing the following operating circs:

From the plate 14 of the oscillator tube by way of lead 4| including blocking condenser 42 to the.

feed-back resistor R4, by way 'of the aligned closed blade contacts of selector switch I024 to the tuned circuit formed of the condenser C4 and the inductance L2 which is connected by leads 43, 44 to the condenser C4,,continuing from the other side of the tuned circuits C4, L2 by way of the closed contact blades and lead 45 to the control grid I of the oscillator tube l2, the grounded cuit Cl, L! by way of leads It, 48 to the controlgrids ii of the two push-pull connected amplifier tubes Ii, so as to deliver amplified 1024 cycle oscillations to the output transformer 35.

In a similar way, oscillations of each of the other testing frequencies may be selectively generated'and delivered to the output transformer 35 by actuating the end spring E- of any one of the other selector switches I28, 156 "92 from the normally open position .to the closed position in the way described above in connection with the oscillating circuit for the 1024 frequency.

The oscillator arrangement described above is distinguished by a number of important characteristics. In designing an oscillator arrangement for producing the required pure sinusoidal tones of frequencies diflering by three tofour octaves of the principalaudible frequency range, practical considerations make it necessary to limit to a minimum the inductances required for the diiferent tuned resonant circuits corresponding to the diflerent generated frequencies. In the audiometer of the invention, only two tuning inductances LI,'L2 are used to produce the different pure tone oscillations in the frequency range between 128 cycles and 8192. cycles. Each inductance. must thus serve with a series of different condensers as a tuned tank circuit of resonant frequenciesfditfering from each other by three octaves. As a result, the impedance of the tank circuit, formed by the inductance in combination with the different condensers greatly increases as the frequency increases. Such large tank 'impedances require, in turn, veryv large feed-back resistors to prevent distortion of the oscillations, and such large feed-back resistors must be accurately adjusted so as to be free from spurious eifects that would interfere with the I proper operation of the oscillation circuits. In

the oscillator arrangement of the invention, these difficulties are overcome, by providing for each tuned'tank circuit a shunting resistance, formed of the two resistors 49 arranged to be connected in parallel to the tuned circuits of the different frequencies so as to limit the maximum impedance across the tank circuit to a value of about 100,000 ohms. Thi in turn, makes it possible to use moderate size feed-back resistors Rl to R1, assuring stable generation of the oscillations of the diflerent frequencies.

By using the diflerent tuned tank circuits required to generate the oscillations of the different frequencies as a'coupling impedance to the push-pull amplifier, the tank acts as a very effective filter for harmonics that might produce distortion. By this arrangement, the oscillatory voltage which is impressed on the control grids of the push-pull amplifier has a very pure wave form and the push-pull amplifier is able 'to deliver oscillations of great purity.

In order to actuate the oscillation generator to selectively generate the desired frequencies, the frequency selector I8 is made in the form of a movable member having, for instance, a cam nose which may be moved into selective en- Gil tact blades from the normally openposition to the closed position, in which the oscillator circuits of the selected frequency are established so as to generate and deliverto the output transformer II the desired frequency, as explained above for the 1024 frequency setting. I Y

The oscillations of the diflerent frequencies are delivered to an attenuator connected to the secondary winding of the output transformer 35.

The attenuator comprises a plurality of series re- I sistances 6i and a plurality of shunt resistance 62 connected between the series resistances 6i and a conductor 3 to form a ladder network provided with taps 84 so that the electric output delivered to the ladder network is logarithmically attenuated, each tap it increasing the attenuation by 5 db.

The different degrees of 65 from tap to tap by means of the attenuation regulator H which has a dial scale 66 with the different degreesof attenuation, so asto indicate gasement with the end blade E of any one of the by an index 81 on the scale it the hearing loss in decibels corresponding to the output delivered by the attenuator network at the tap N engaged by the sliding contact 85, the attenuation regulator H being movable in the direction of the arrow, and back, to bring the sliding contact into contact engagement with any one of the tape it of the attenuator network.

An air conduction receiver .10 and a bone con duction receiver II are arranged to be connected between the attenuator lead 63 and a switch I:

which is connected through lead II to sliding contact 65 to enable energization of either one of the receivers with the attenuated oscillatory output to produce sound energyof the frequency determined by the frequency selector lOf'and of an intensity level determined by the attenuation regulator i I.

- The attenuator scale is arranged to indicate, in terms of hearing loss, for eachattenuator setting, the intensity of the sound produced by the receiver, in decibels above the intensity of sound which produces a barely audible sound, that -'.is, above the sound intensity of the lower threshold of hearing, as indicated on the lower curve of Fig. 2, which is used as the zero sensation level for the attenuation scale. The scale is calibrated in 5 db. steps.

For satisfactory operation, the audiometer should be able to impress on thetested hearing organ a pure tone of the frequency indicated on the frequency selector scale, ,and to indicate on a simple attenuation scale arrangement the intensity of the sound delivered by the receiver with respect to a zero. represented by the sound ener y level on the attenuation scale, varies unequally in opposite directions while the frequency is varied from one end of itsscale to its other end, drop ping by about decibels to a minimum as the frequency is increased from the lower limit of attenuation are ad iusted or regulated by moving a sliding contact per second, and rising again by about 90 decibels above the minimum as the frequency is further transformer 30 to the attenuator so that for each of the dlil'erent testing frequencies, the attenuator scale gives an accurate indication of the increased to the upper limit of the audible frequency range. Furthermore, the efllciency or sensitivity of the available receivers differs greatly over the frequency range of audible hearing, being usually a maximum at about thousand cycles, and falling oil at lower and higher frequencies.

According to the invention, the difliculties connected with complicated attenuator scale arrangements of prior audiometers are eliminated, and an audiomcter operating with a single attenuation scale and indicator is made possible, by combining the oscillator frequency control means and the attenuator control means with compensating means arranged to be actuated by the frequency control means to unequally modify in opposite directions the energy volume supplied by the oscillator to the receiver so as to compensate for irregularly unequal variations of the sound energy giving zero level as well as any other irregularities occurring in the operation of the audiometer while the frequency of the oscillations is varied from one end to the other end of i the frequency scale.

One type 'of such audiometer arrangement, which correlates the operation of the frequency selector with the attenuation regulator and the control of the intensity of the sound delivered by the receiver in the way described above, is shown diagrammatically in Fig. 3. It comprises a plurality of compensating resistances Al to Al and BI to B1 interconnected to a lead 15 that is connected to the lead 63 from one end of the secondary winding of the output transformer 35 so that the individual compensating resistances Al Bl may be connectedin shunt to the attenuator 60. As indicated diagrammatically in Fig. 3, the other ends of the resistors Al to Al and BI to B1 are arranged to be selectively connected by the contact blades SA of the selector switches I28 8l92 to leads 16, ll, which, in turn, are arranged to be connected by switch 18 to the lead 19 extending from the other end of the second ary winding of the output transformer 35. The switch 18 is operated conjointly with switch 12 which controls the connections of the air conduction receiver or bone conduction receiver II to the attenuator so that lead 10, which com;

pletes the connection to the resistors Al to A1, is connected to the output transformer 35 when the air conduction receiver. is energized, and lead TI, which completes the connection to the resistors Bl to Bl is connected to the output transf'ormer 30 when the bone conduction receiver II' is energized.

With this arrangement, each setting of the frequency selector automatically connects one of the compensating resistors Al to Al or one of the equalizing resistors Bl to B1 in shunt to the secondary winding of the output transformer 35 while the bone conduction or air conduction re- A celver, respectively, is energized by the attenuator. The individual compensating or equaliz ing resistors Al to A1 are adjusted to make the electric output delivered by the output transformer 35 to the attenuator such that for each of the different testing frequencies, the attenuator scale gives an accurate indication of the sound intensity delivered by the air conduction receiver in terms of the hearing loss. Similarly, the individual resistors Bl to B1 are adjusted to equalize the electric output delivered by output sound intensity delivered by the bone conduction receiver.

As shown in Fig. 3, the'tunction or the equaii'zing resistors Al to A1 and BI to B1 is to reduce the intensity of the oscillations delivered to the input side of the attenuator and thus compensate for the diflerences of the sensitivity of the receiver and also for variations in the characteristics of the oscillator at the different frequencies of the audiometer range. Obviously, such resistance, or, in general, any impedancetype equalizing network effects equalization of electrical output energy only by reducing the maximum output. In other words, the electrical output delivered by the oscillation generator must be at least as large as the maximum output required by the receiver under its most unfavorable operating conditions.

However, even the best available air conduction receivers, which must be used in all fundamental audiometric measurements, have very little sensitivity in the low frequency range below about 500 cycles, and have also substantially reduced sensitivity in the high-frequency range above about 4000 cycles. Since a satisfactory audiometer should, in general, be able to explore 80% of the hearing loss of a person in the principal audible frequency range between 128 and 8192 cycles, this means that the minimum output of the receiver must be about 100 db. above the lower threshold of normal hearing in the range of about 1000 cycles where the hearing range is a maximum. -An audiometer arranged to operate in accordance with the principles described above would, accordingly, have to be able to deliver a receiver output of 100 db. above the threshold of'hearing throughout the frequency range between about 100 cycles to 8000 cycles. This would require not only an oscillation generator, of abnormally large output capacity, but also very powerful receivers able to deliver such output in the low frequency range between 100 would be modified each time they are subjected to such abnormal power output duty.

To overcome these difliculties, and to assure that for each setting of the attenuator, the intensity of the sound delivered by the receiver is accurately indicated on the attenuator while the setting of the frequency selector is changed over the frequency range, the operation of the frequency selector is automatically correlated with the attenuation scale arrangement to assure that if the frequency selector is shifted, from-a position in the mid-frequency range between about 400 to 2500 cycles in which the receiver is able to deliver a db. output to a position in marginal frequency ranges in which the receiver is unable to deliver the maximum output of the 100 db. sound intensity, the attenuation index on the attenuator scale is automatically brought to a position which makes it impossible to make on the attenuation scale a reading indicating a sound intensity greater than that which the receiver is able to deliver.

One form of arrangement for securing such automatic correlation 'of the frequency selector 2,257,262 and the attenuator is shown diagrammatically in Fig. 3. The frequency selector l has a cam" member 80arranged to cooperate with a cam follower 8| which holds the attenuation scale index 61. The cam follower 8| is shown guided in sleeves 82 and is urged by a biasing spring 88 against the cam surface of the cam member 88 so as to slide thereon and be brought into'engagement with different portions of the cam surface while the frequency selector is moved to the different positions indicated by thefrequency index 52 on the frequency scale In the exemplification of the invention shown diagrammatically in Fig. 3, the camsurface of thecam member 80 of the selector is so arranged that as long a the frequency setting of the frequency selector remains within the mid-frequency range, the index 61 is retained by the cam follower 8| in a fixed position so as to indicate on the audiometer the maximum extent of the attenuation'required to determine a hearing loss of 80% in this frequency range, that is, up to about 100 db. To this end, the cam member 88 is provided with a cam surface portion 85 which isso correlated to the frequency scale 5| and the frequency indicator 52, that as long as the frequency selector is set to generate a frequency-within the range at which the receiver is able to deliver 100 db. above the threshold of hearing, for instance-in the case shown, between about 500 and 2000 cycles, the cam follower 8| with its attenuator index 61 will remain fixed in the position at which the attenuator scale is able to indicate the maximum hearing loss shown on its scale, in the case illustrated, 100 db.; and that as soon as the frequency selector I0 is moved to a position where its'index indicates on the frequency scale a frequency at which the receiver is not able to deliver sound intensity corresponding to a 100 db. hearing loss, such as at a frequency between 200 to 350 and between 3000 to 5000 cycles, the cam follower 8| is brought into engagement with the cam surface portions 86, 81, respectively, thereby ent frequency ranges and depending on the maximum output that the receiver is able to deliver at the different frequency ranges, the difierent cam surface portions may be arranged to permit the operation of the attenuator to indicate the maximum output that the receiver can deliver at the given frequency;

It is likewise obvious that if the audiometer of the type described above were required to explore a maximum loss of hearing of 75 db. above the normal threshold of hearing throughout the entire frequency range between 100 and 8000 cycles, the cam surface of the cam member 80 would be designed to keep the index 61 at all times in a fixed position where the maximum attenuation that can be indicated on the attenuator is not more than the maximum receiver output at the frequency of its lowest sensitivity, for instance, only 75 db. so as to make it impossible to make erroneous audiometric measurements.

In order to facilitate the making of audiometric measurements, the audiometer of the invention is also provided with a tone interrupter arrangement which enables momentary interruption of a tone without producing a click or any other noise that might mislead the individual who is being tested.

The interrupter arrangement comprises an interrupter switch 8|, which is normally in its back contact position, and a resistor 82 connected in series with the lead which applies the operating biasto the screen grids ll of the amplifier tubes l6 so as to maintain them at their normal operating potential. When the interrupter switch 8| is actuated from the position shown in Fig. 8

shifting the attenuation index to a position in l which the attenuator scale can indicate only the maximum output that the receiver is able to actually deliver at such marginal frequencies, in the case illustrated, 90 db. hearing loss on the attenuation scale 88; and that on moving the selector to a marginal frequency range below 200 frequency range corresponding to the setting of the frequency selector, in the case illustrated, 75 db. hearing loss.

The several cam surface elements 85 to 88 are so arranged that, as the frequency selector is moved to the different frequency settings indicated on the frequency dial, the cam follower 8| which fixes the position of the attenuation index,

or, in general, controls the attenuation scale arrangement, automatically moves from its engagement with one cam surface portion into engagement with another cam surface portion corresponding to the different frequency range settings of the frequency selector. Obviously, depending on the sensitivity of the receiver in the difierto the front contact position, it opens the connection of the screen grids ll to the positive bias terminal .+B and connects the screen grids to the ground and to a shunting condenser 83 which in the normally closed position of switch 9| is maintained fully chargedto the normal screen grid potential. As a result, actuation of the interrupter switch 8|, to disconnect the screen grids l5 from the +3 potential, does not instantaneously apply the ground potential to the screen grids, but the condenser 88 which is fully charged to the screen grid potential only gradually discharged across the resistance 82, thereby gradually reducing the bias of the screen grids |5 so that the tone dies away slowly without causing any clicks or other masking noises that would disturb the individual under test and affect the results of his tests. Restoration of the tone, by returning the interrupter switch 8| to the back contact position shown in Fig. 3, does not suddenly apply the full screen potential to the screen grids I], because during the initial period while the condenser 83 is being recharged, the potential applied to the screen gradually rises and the level of the tone gradually rises without producing any disturbing masking noises or clicks.

The, switch controlled signal light 28 makes it possible for the person under test to indicate the moment when he ceases to near the sound. The energizing circuit of lamp 28 is provided with a signal switch which is manually operated by the person under test who is usually instructed to release and press the switch when he and ceases to hear, respectively.

In order to make it possible to converse with a. deafened person who is undergoing a test, the push-pull amplifier associated withthe tubes l8 for amplifying the test oscillations may be also hears utilized to amplify speech directed to the person under test. To this end, a microphone SI and a multi-blade control switch 95 may be arranged in the way shown in Fig. 3 to connect the microphone through the input transformer 96 so as to impress on the control grids ii of the two ampliiier tubes It the speech current transmitted by the transmitter 94 when the switch 95 is moved from the normal position shown in Fig. 3 to the position in which the microphone 94 is connected to the secondary winding of the transformer 96 and the control grids of the amplifier tubes l6 are' disconnected from their normal operating circuits and connected to the secondary winding ofthe'transformer 95. The primary winding of thetransformer 96 is shown connected by means of a network including-a choke coil 91 and a condenser 98 connected across the resistor 31 so as to supply to the transmitter microphone 04 a D. C. exciting current.

Fig. 1 illustrates the mechanical features of the actual construction of a frequency selector and attenuation regulator for the audiometer of the type described in connection with Fig. 3. In this construction, the frequency selector I and the attenuation regulator II are shown in the form of rotatable shafts to which are afllxed the dial members with the frequency scale 5i and the attenuator scale 66. As shown in Fig. 1, the attenuator scale is graduated not only for the air conduction range extending from to 100 db. hearing loss, as indicated in Fig. 3, but alsobears a corresponding scale for the bone conduction receiver running from to db. so that when the tests arperformed with the air conduction receiver, the 20 to 100 db. scale is read, and when the tests are conducted with the bone conduction receiver, the -30 to 60 db. scale is read,

The cam 80 and the cam follower 8| used in the construction shown in Fig. 1 are suitably arranged in accordance with the principles described in connectionwith Fig. 3. The cam follower is shown interconnected by a pivotal pin with the index arm 61 that is pivotally mounted on the selector shaft I I and carries an index: projection 61, a follower roller 105 pivotally mounted on the follower 8| being maintained in engagement with the diiferent cam surfaces of the cam member 80 in each position of the frequency dial The cam portions of the cam 80 which are effectiv in fixing the position of the attenuator index are indicated by the arrows I28 to 8l92 shown on the cam 80 in Fig. 1. As shown in Fig. 1, the panel I00 of the audiometer has also mounted thereon the line switch 22, the meter 26, the pilot lamp 30, and the signal lamp 29.. In addition, there is also provided a Jack I 07 for connecting the microphone to its operating circult, and a Jack I08 for connecting the receiver,

the receiver ack and receiver plugs being combined with suitable spring contacts actuated by the insertion of the jacks of the respective receivers to automatically perform the functions of switches 12, 18 when the respective plugs of the air conduction and bone conduction receivers are inserted into the jacks. The principles of the invention described above in connection with specific exemplifications' thereof will suggest to those skilled in the art many other modifications thereof. '='1,

It is accordingly desired t'hat the appended claims be given a broad construction commensurate with the scope of the invention within the art.

I claim: I

1. In an audiometer, an electric oscillation generator, a frequency selector including a frequency scale for selectively determining the frequency of the oscillations in accordance with the position of the frequency selector as indicated on the frequency scale, a receiver, an attenuator network connected between the oscillationgenerator, and an attenuation regulator including an attenuation scale for adjusting the degree of the attenuation of the sound output of the receiver in accordance with the position of the attenuation regulator as indicated on the attenuation scale, said frequency selector, said network, and said attenuation regulator being so arranged and correlated as to enable accurate regulation of the sound intensity delivered by the receiver at each selected frequency with a single attenuation scale.

2. In an audiometer, an electric oscillation generator, a frequency selector including a frequency scale for selectively determining the frequency of the oscillations in accordance with the position of the frequency selector as indicated on the frequency scale, a receiver, an attenuator network connected between the oscillation generator, and an attenuation regulator including an attenuation scale for adjusting the degree of th attenuation of the sound output of the receiver in accordance with the position of the attenuation regulator as indicated on the attenuation scale, said frequency selector, said network, and said attenuation regulator being so arranged and cor-.

related that the sound intensity delivered by the receiver is maintained substantially at the value indicated on the attenuation scale while the frequency of the tone is varied over a selected part of the audible frequency range.

3. In anaudiometer, an electric oscillation generator, a frequency selector including a frequency scale for selectively determining the frequency of the oscillations in accordanc with the position of the frequency selector as indicated on the frequency scale, a receiver, an attenuator network connected between the oscillation generator, and an attenuation regulator including an attenuation scale for adjusting the degree of the attenuation of the sound output of the receiver in accordance with the position of the attenuation regulator as indicated on the attenuation scale,

said frequency selector, said network, and said attenuation regulator being so arranged and correlated as to permit at least 80% exploration of the loss of useful hearing of a person in the midfrequency range of audible hearing as well as in the lower and upper parts of the audible frequency range between about 100 and 8000 cycles.

4. In a device for testing the hearing: the combination of an electric oscillation sourc element for supplying one of a plurality of predetermined different fixed frequencies separated by predetermined intervals of the audio-frequency range in-' cluding frequency control means having an adjustable frequency dial for controllably adjusting in predetermined steps corresponding to said inf tervals the frequency of the oscillations in accordance with the indication on the frequency dial; a receiver element for converting the electric energy of said source into sound; and an attenuator device connecting said receiver to said source and having attenuation control means including an adjustable attenuation dial for conaccuses cam hearing level representing the minimum sound energy barely audible .to a normal person and varying unequally overthe frequency range between a minimum value at an intermediate frequency and higher values in the frequency ranges below and abov said intermediate frequency; said attenuation control means being coupledto and subject to the control by said frequency dial and having means actuated to unequally modify in opposite directions said energy volume in diflerent parts of the frequency range and compensate for the unequal variations of the sound energy giving the zero hearing level while the. frequency is variedfrom one end to the other end of the frequency dial for preventing disturbance of any desired adjustment of said attenue ator device. 7

5. A device as specified in claim 4, said attenuatmdevice being adjustable independently and having a single attenuation scale on its dial.

6. Ardevice as specified in claim 4,- comprising a variable resistance in shunt of said attenuator device and operable by th frequency dial and constituting the means for controlling the sound energy. 7

'1. In a device for testing the hearing: the combination of an electric oscillation source element for supplying one of a plurality of predetermined different fixed frequencies separated by predetermined intervals of the audio-frequency range including a frequency dial and an oscillator knob adjustable for controlling in predetermined steps corresponding to said intervals the frequency of the oscillations in accordance with the indication on the frequency dial; a receiver element for converting the electric energy of said source into sound; an attenuator rheostat with an attenuation dial anda rotary attenuator knob therefor for controllably adjusting the'energy volume supplied to said receiver in accordance with the indication on the attenuation dial; said attenuation dial being calibrated to indicate energy variations from a zero hearing level representing the minimum sound energy barely audible to a normal personand varying unequally over the frequency range between a minimum value 7 at an intermediate frequency and higher values in the frequency ranges below and above said intermediate frequency, an adjustable rheostat in shunt to said attenuator rheostat, and means operated by said oscillator knob to operate said shunt rheostat to unequally modify in opposite directions said energy volume in different parts of the frequency range and compensate for the unequal variations of the sound energy giving the zero hearing level whiie the frequency is varied from one end to the other end of the frequency dial so as to prevent disturbing said attenuator rheostat. l

8. An audiometer device for testing the hearing comprising: an electric oscillation source element for supplying one of a plurality of predetermined different fixed frequencies separated by predetermined intervals of the audio-frequency range including frequency control means having a. frequency dial for controllably adjusting in predetermined steps corresponding to said intervals the frequency of the oscillations in accordance with receiver element for converting the energy of said the indication on the frequency dial; a a

source into sound; and an attenuation control device vconnecting said receiver to said source, including adjusting means having an attenuation'dial for adjusting the energy volume supplied to said receiver in accordance with the indication on the attenuation dial; said attenuation dial being calibrated to indicate ener y variations from a zero hearing level representingthe minimum sound energy barely audible to anormal person and varying unequally hev tween a minimum value at an intermediate frequency and higher values in the frequency ranges below and above said-intermediate frequency; in combination with means actuated by the iref quency control means to unequally modify in opposite directions said energy volume in difiercut parts of the frequency range while the frequency is varied from one end to the other end of the frequency dial so that the frequency control dial serves also to control the volume of the energy and compensate for the unequal variathe sound energy giving the zero hearing tions of level.

9. A deviceas specified in claim 8, comprising means forrcontrolling the energy volume by the adjustment of the frequency control dial, while preventing any disturbance of the indicated volume on the attenuation dial. l

10; In an audiometer instrument for testing the hearing: the combination of an electric oscillation source element for supplying one of a plurality of predetermined different fixedfrequencies separated by predetermined intervals of the audio-frequency range including a frequency control having a graduated frequency scale with a frequency indicator for traversing the graduated path of travel by a relative moveuated attenuation scale.

ment between the scale and indicator and con-=- trollably adjusting in predetermined steps corre spending to said intervals the frequency of the oscillations in accordance with the indication on the scale; a receiver element for converting the electric energy of said source into sound; an at- 'tenuation control device connecting said receiver to said source including attenuator adjusting means having a graduated attenuation scale with an attenuation indicator for traversing the graduated path of travel thereof by relative movement between said attenuation indicator and said attenuation scale and adjusting the energy volume supplied to said receiver in accordance with the indication on the attenuation scale; said attenuation scale being calibrated to indicate energy variations from a zero hearing level representing the minimum sound energy' barely audible to a. normal person and varying unequally over the frequency range between a minimum value at an intermediate frequency and higher values in the frequency ranges below and above said intermediate frequency; and instrumentalities inter-relating said frequency control and said attenuation control device to provide attenuation control by the adjustment of the frequency control and actuated to unequally modify in opposite directions said energy volume in different parts of the frequency range and compensate for the unequal variations of the sound energy giving the zero hearing level while the frequency is varied from one end to the other end of the frequency scalefor preventing any disturbance of the indicated reading on the grad- HEIMAN w. KOREN. 

